Telephone line test equipment for use by telephone subscribers, as distinguished from telephone company (TELCO) test equipment, is a relatively new field in the communication art. U.S. Pat. No. 4,467,148, Stafford et al., assigned to the present assignee, discloses this new genre of test equipment and specifically discloses the ability to test telephone tie or trunk lines with the use of a trunk selectable private branch exchange (PBX). The telephone line analyzer disclosed in the '148 patent although capable of testing an inward dial WATS line as well as other DID lines, would be unable to identify such lines if a plurality of such lines are present at one user location.
Typically, large corporations, reservation centers, order entry centers, and others that need many inbound trunks for its customers or employees rent a plurality of inbound trunk lines all of which are usually associated with a single telephone number. Thus for a reservation system, a single 800 number would typically be listed for access to the reservation system. Depending upon the number of anticipated callers at any given time, a plurality of inward dial WATS lines (INWATS) are leased from the appropriate telephone company. The customer, when calling the 800 number, is routed to one of these inbound trunks by the telephone company routing system. One such routing system is known as a rotary hunt wherein the telephone company routes inward dialed calls in a circular pattern. For instance, if the subscriber has 20 INWATS lines, the first call would be routed to INWATS line #1, the next call to INWATS line #2, etc., up to INWATS line #20, for the twentieth caller, and back to INWATS line #1 for the twenty-first caller, etc.
In a "most available first" hunt routine, the telephone company routes inward dialed calls to the lines on a prioritized basis based upon the least used lines with respect to the calls received on the previous day. Thus, for instance, if the subscriber again has 20 INWATS lines, and if on day 1 line #3 was the most heavily used, line #7 the next heaviest used, etc., while lines #5 and #10 were the next to the least and the least used, with the remaining lines being somewhere in between these extremes, the inward dialed calls would be routed on a sequential basis to line #10, then line #5, etc. Line #7 and line #3 would be selected for the nineteenth and twentieth calls respectively while line #10 would be selected for the twenty-first call. This prioritization is then repeated for the following day based upon the usage of the lines for the present day. This routing routine is similar to the circular hunt routine in that a set sequence of lines is used on any given day.
In a third type of routing routine the telephone company routes calls in what is known as a top down rotary sequence. In this routine, a selected sequence for the subscriber's lines is chosen; such as line #1, line #2, . . . line #20 (for a twenty inbound trunk example), back to line #1, etc. However, if for example the last call was placed on line #5 and before the next incoming call line #2 has become available (the calling party on line #2 has "hung up"), then line #2 and not line #6 is selected for the next call.
For the "circular" hunt and "least used first" hunt routines the trunk identification system "knows" that each inbound trunk will be accessed by the telephone company routing routine although the actual line to be selected for any given call is not known by the trunk identification system. Therefore in the example where the subscriber has leased twenty inbound trunks, a test program conducted during off-hours (such as in the early morning when the lines are not otherwise being used) will access each inbound trunk without the need for "busying out" the trunks which have been tested.
In the "top down rotary" hunt routine the first trunk in the sequence would be repetitively accessed by the routing system if that line was not kept in an "off hook" condition ("busy out") after being tested. Thus the trunk identification system not only must identify such a tested inbound trunk, but must also maintain it in an "off-hook" state by "busying" it out after the test routine. This "busy out" is performed on each tested trunk until all trunks have been tested.
The only prior art trunk identification unit known is that of the present assignee and is identified as the JANUS.TM. Trunk Identification Unit. This prior art device, however, uses analog phase lock loop circuitry for the detection of a particular DTMF tone and does not use or suggest any multiplexing capability for the phase lock loop circuitry resulting in separate phase lock loop detection circuits installed for each inbound trunk to be tested. This analog trunk identification unit has been found to be subject to drift as a result of ambient temperature variations as well as due to aging of circuit components. As a result, the trunk identification unit can potentially falsely interpret the presence of the unique DTMF tone (normally the "D" DTMF tone) and then transmit its identification information regarding this particular trunk line even though the inbound trunk is not connected to test equipment at that time. Similarly, due to the same inaccuracies associated with the analog DTMF detection circuitry, the actual presence of a "D" tone can be misinterpreted, thereby resulting in the trunk identification system failing to transmit the identification information, resulting in negation of any value with regard to the test of that INWATS or DID line
In contrast, the improved trunk identification system of the present invention employs digital DTMF decoders used in a multiplexed fashion so that eight DID or INWATS lines can be tested by two digital DTMF decoders, receivers. This arrangement results in accurate decoding of the initiating "D" tone when present and not falsely detecting a "D" tone during normal voice communications. Furthermore, due to the fact that the DTMF digital receivers are crystal controlled, drift associated with changes in ambient temperature or aging of components is essentially eliminated.